Hot water priority control

How a heat pump pauses or reduces space heating while it reheats a domestic hot-water cylinder.

Hot water priority directs a heat pump’s output to the domestic hot-water cylinder when the cylinder needs reheating. In many hydronic systems a valve or pump arrangement temporarily stops space-heating flow while the cylinder coil is supplied at a higher temperature.

The pause is deliberate. It lets one heat pump serve two duties without trying to provide two different water temperatures through the same flow at once.

The control sequence

A typical sequence is:

The cylinder sensor falls below its reheat threshold or a schedule starts a hot-water period.
The controller moves a three-port valve or changes pump operation.
The heat pump raises its leaving-water target for cylinder charging.
The cylinder reaches its target or a time or fault limit is reached.
The valve returns and space heating resumes on its weather-compensated target.

The exact sequence, sensor logic and valve position are product-specific. Some systems can serve duties differently or use separate heat pumps.

Why space heating can pause

Domestic hot water usually needs a higher heat-pump flow temperature than low-temperature emitters. Running the radiators and cylinder together at the hot-water temperature can reduce efficiency and overheat rooms. Splitting flow between them can also make cylinder reheat slow or leave emitters short of flow.

Priority simplifies control and gives the cylinder the heat pump’s available output. The building fabric and emitters carry the space-heating demand during the temporary pause.

There is no universal acceptable pause duration. It depends on cylinder volume and coil, starting temperature, heat-pump output, hot-water draw and building heat loss.

Cylinder and coil selection determine recovery

A heat-pump cylinder needs a heat exchanger suited to lower-temperature, high-flow operation. An old boiler cylinder can have a small coil that restricts heat transfer, forcing hotter water or a long recovery.

MIS 3005-D requires the hot-water cylinder heat exchanger to follow heat-pump or cylinder manufacturer recommendations. The desired reheat time must be agreed with the customer.

The design record should state:

usable cylinder volume and target
coil or heat-exchanger performance at the proposed conditions
heat-pump output during hot-water operation
expected recovery basis
immersion or supplementary-heater role

A headline cylinder capacity alone does not predict reheat time.

Current MCS temperature capability

As at 22 July 2026, MIS 3005-D Issue 3.0 requires a heat pump serving a domestic hot-water cylinder to be capable of at least 55°C flow at the design conditions. This is a product and system capability requirement from the dated standard.

It does not mean space heating should run at 55°C or that the stored-water hygiene strategy is automatically satisfied. Cylinder storage, pasteurisation and scald protection remain separate design decisions.

Sensor position and hysteresis

The cylinder sensor measures one location, while hot water stratifies. A high sensor can report hot even when little usable volume remains; a low sensor can call for heat after a small draw. The sensor pocket and control method should follow the cylinder and heat-pump design.

Hysteresis is the difference between the reheat start and stop conditions. Too narrow a band can cause frequent short reheats. Too wide a band can leave the user with insufficient hot water. Set it from product guidance and household service needs rather than a generic number.

Scheduling

Hot-water periods can be placed where they do least harm to comfort or cost. Consider:

expected draw times
cylinder standing loss
tariff periods
outdoor conditions and space-heating recovery
solar or battery availability
pasteurisation schedule

One sufficiently planned reheat can be more stable than repeated small calls, but the suitable pattern depends on the cylinder and household. Do not suppress reheating so aggressively that hygiene or usable hot water is compromised.

Immersion and pasteurisation

An immersion may finish a high-temperature hygiene cycle, provide backup after a fault or support recovery beyond the heat pump’s efficient range. Its energy should be visible separately from compressor electricity where possible.

Hot-water priority and pasteurisation are not the same. Priority selects the cylinder service; pasteurisation follows the system’s bacterial-risk control. Do not disable a hygiene cycle simply because it uses direct electricity.

Common faults

Space heating does not resume

Possible causes include a stuck or miswired valve, sensor that never reaches target, excessive target, air or low flow through the coil, or a controller still in hot-water override.

Hot water reheats slowly

Check heat-pump output at the required temperature, coil sizing, primary flow, strainer condition, sensor placement, scale and whether another circuit is taking flow.

Rooms cool noticeably

The hot-water period may be too long or frequent, the building may have little thermal reserve, or the heat pump and cylinder coil may be poorly matched. Raising the space-heating flow afterwards is not the only remedy; fix the recovery duty and schedule first.

Unexpected direct-electric use

Review immersion enable conditions, hygiene cycles, fault fallback and whether the heat pump reached its limit. A meter or controller log is better evidence than the cylinder feeling hot.

Commissioning

Test a complete cylinder call and return to heating. Record:

sensor readings and target
valve and pump sequence
heat-pump flow and return temperatures
primary flow through the cylinder circuit
start and finish conditions
compressor and immersion energy where available
space-heating restart

The owner should be shown normal scheduling, boost, holiday mode and the difference between a one-off boost and an installer setting.